Symptoms and Signs
If humid conditions prevail during the maturation period of pistachio fruit, several fungi can colonize and decay pistachios, although infection by Aspergillus spp. can occur under drier than normal conditions.
Alternaria causes deterioration of fruit epicarp (see also ALTERNARIA LATE BLIGHT DISEASE). Small or large black lesions develop that may expand over the entire hull surface. Depending on the degree of colonization, A. alternata can invade the kernel, causing kernel discoloration and decay. Whether or not the kernel is infected, Alternaria can damage the quality of the nut by causing brown to black stain marks on the shells.
Aspergillus niger causes a blight that turns the hulls light beige to yellow. If hulls are removed, the black, powdery sporulation of the fungus is evident under the hull and on the surface of the shell. No characteristic or typical symptoms develop from infections by the other fungi listed as casual agents of these molds, although such infections will stain shells and causes hulls to adhere to shells. After shelling, however, characteristic sporulation by these fungi can be obvious by the naked eye.
Comments on the Disease
Colonization of nuts by Aspergillus molds is often associated with insect infestations. Infection of the nuts by A. flavus may result in contamination with aflatoxin(s). Dry, hot weather favors infection by Aspergillus spp.
Take preventive measures during the period that the fruit are maturing.
- Avoid water stress during mid-May when the shell is growing rapidly to reduce the incidence of early shell split and control infestations of navel orangeworm.
- Control navel orangeworm to reduce infection by A. flavus and A. parasiticus and reduce aflatoxin contamination.
- When establishing an orchard, select rootstocks that are not conducive to high incidence of early splits (e.g., UCB1).